diff --git a/.gitignore b/.gitignore
index 889d9e6..2c2e6ad 100644
--- a/.gitignore
+++ b/.gitignore
@@ -10,3 +10,7 @@ Makefile
 CMakeCache.txt
 src/zopt.h
 src/zopt.c
+lexer.c
+lexer.h
+parser.c
+parser.h
diff --git a/lib/stack.c b/lib/stack.c
new file mode 100644
index 0000000..8dfe0db
--- /dev/null
+++ b/lib/stack.c
@@ -0,0 +1,40 @@
+#include "stack.h"
+#include "../lib/xalloc.h"
+
+#include <stdlib.h>
+
+struct stack {
+	size_t max_size;
+	size_t cur_size;
+	void** arr;
+};
+
+stack_t* alloc_stack(size_t size)
+{
+	stack_t* stack = xmalloc(sizeof(stack_t));
+	stack->arr = xcalloc(size, sizeof(void*));
+	stack->max_size = size;
+	stack->cur_size = 0;
+	return stack;
+}
+
+void free_stack(stack_t* stack)
+{
+	xfree(stack->arr);
+	xfree(stack);
+}
+
+void push(stack_t* stack, void* elt)
+{
+	if (stack->cur_size == stack->max_size) {
+		stack->max_size *= 2;
+		xrealloc(stack->arr, stack->max_size);
+	}
+	stack->arr[stack->cur_size++] = elt;
+}
+
+void* pop(stack_t* stack)
+{
+	void* res = stack->arr[--stack->cur_size];
+	return res;
+}
diff --git a/lib/stack.h b/lib/stack.h
new file mode 100644
index 0000000..9417238
--- /dev/null
+++ b/lib/stack.h
@@ -0,0 +1,15 @@
+#ifndef ZMAP_STACK_H
+#define ZMAP_STACK_H
+
+#include <stddef.h>
+
+struct stack;
+typedef struct stack stack_t;
+
+stack_t* alloc_stack(size_t size);
+void free_stack(stack_t* stack);
+
+void push(stack_t* stack, void* elt);
+void* pop(stack_t* stack);
+
+#endif /* ZMAP_STACK_H */
\ No newline at end of file
diff --git a/lib/xalloc.c b/lib/xalloc.c
new file mode 100644
index 0000000..60e73e7
--- /dev/null
+++ b/lib/xalloc.c
@@ -0,0 +1,44 @@
+#include "xalloc.h"
+#include "../lib/logger.h"
+
+#include <stdlib.h>
+
+void die() __attribute__((noreturn));
+
+void* xcalloc(size_t count, size_t size)
+{
+	void* res = calloc(count, size);
+	if (res == NULL) {
+		die();
+	}
+	return res;
+}
+
+void xfree(void *ptr)
+{
+	free(ptr);
+}
+
+void* xmalloc(size_t size)
+{
+	void* res = malloc(size);
+	if (res == NULL) {
+		die();
+	}
+	return res;
+}
+
+void* xrealloc(void *ptr, size_t size)
+{
+	void* res = realloc(ptr, size);
+	if (res == NULL) {
+		die();
+	}
+	return res;
+}
+
+void die()
+{
+	log_fatal("zmap", "Out of memory");
+}
+
diff --git a/lib/xalloc.h b/lib/xalloc.h
new file mode 100644
index 0000000..78cd278
--- /dev/null
+++ b/lib/xalloc.h
@@ -0,0 +1,14 @@
+#ifndef ZMAP_ALLOC_H
+#define ZMAP_ALLOC_H
+
+#include <stddef.h>
+
+void* xcalloc(size_t count, size_t size);
+
+void xfree(void *ptr);
+
+void* xmalloc(size_t size);
+
+void* xrealloc(void *ptr, size_t size);
+
+#endif
\ No newline at end of file
diff --git a/src/.gitignore b/src/.gitignore
new file mode 100644
index 0000000..87de78c
--- /dev/null
+++ b/src/.gitignore
@@ -0,0 +1,4 @@
+lexer.c
+lexer.h
+parser.c
+parser.h
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index a5c6346..95bd9ab 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -12,6 +12,8 @@ SET(LIB_SOURCES
 	${PROJECT_SOURCE_DIR}/lib/pbm.c
 	${PROJECT_SOURCE_DIR}/lib/random.c
 	${PROJECT_SOURCE_DIR}/lib/rijndael-alg-fst.c
+	${PROJECT_SOURCE_DIR}/lib/stack.c
+	${PROJECT_SOURCE_DIR}/lib/xalloc.c
 )
 
 # ADD YOUR PROBE MODULE HERE
@@ -40,7 +42,9 @@ SET(PROBE_MODULE_SOURCES
 SET(SOURCES
 	aesrand.c
 	cyclic.c
+	expression.c
 	fieldset.c
+	filter.c
 	get_gateway.c
 	monitor.c
 	recv.c
@@ -50,6 +54,8 @@ SET(SOURCES
 	zmap.c
 	zopt_compat.c
 	"${CMAKE_CURRENT_BINARY_DIR}/zopt.h"
+	"${CMAKE_CURRENT_BINARY_DIR}/lexer.c"
+	"${CMAKE_CURRENT_BINARY_DIR}/parser.c"
 	${EXTRA_PROBE_MODULES}
 	${EXTRA_OUTPUT_MODULES}
 	${PROBE_MODULE_SOURCES}
@@ -69,6 +75,14 @@ add_custom_command(OUTPUT zopt.h
 	COMMAND gengetopt -C --no-help --no-version --unamed-opts=SUBNETS -i "${CMAKE_CURRENT_SOURCE_DIR}/zopt.ggo" -F "${CMAKE_CURRENT_BINARY_DIR}/zopt"
 	)
 
+add_custom_command(OUTPUT lexer.c
+	COMMAND flex -o "${CMAKE_CURRENT_BINARY_DIR}/lexer.c" --header-file="${CMAKE_CURRENT_BINARY_DIR}/lexer.h" lexer.l
+	)
+
+add_custom_command(OUTPUT parser.c
+	COMMAND byacc -d -o parser.c parser.y
+	)
+
 add_executable(zmap ${SOURCES})
 
 target_link_libraries(
diff --git a/src/expression.c b/src/expression.c
new file mode 100644
index 0000000..537be78
--- /dev/null
+++ b/src/expression.c
@@ -0,0 +1,162 @@
+#include "expression.h"
+#include "fieldset.h"
+
+#include "../lib/xalloc.h"
+
+/* Static helper functions */
+
+static node_t* alloc_node();
+static int eval_gt_node(node_t *node, fieldset_t *fields);
+static int eval_lt_node(node_t *node, fieldset_t *fields);
+static int eval_eq_node(node_t *node, fieldset_t *fields);
+static int eval_lt_eq_node(node_t *node, fieldset_t *fields);
+static int eval_gt_eq_node(node_t *node, fieldset_t *fields);
+
+
+static node_t* alloc_node() 
+{
+	node_t *node = xmalloc(sizeof(node_t));
+	memset(node, 0, sizeof(node_t));
+	return node;
+}
+
+static int eval_gt_node(node_t *node, fieldset_t *fields)
+{
+	int index = node->left_child->value.field.index;
+	uint64_t expected = node->right_child->value.int_literal;
+	uint64_t actual = fs_get_uint64_by_index(fields, index);
+	return (actual > expected);
+}
+
+static int eval_lt_node(node_t *node, fieldset_t *fields)
+{
+	int index = node->left_child->value.field.index;
+	uint64_t expected = node->right_child->value.int_literal;
+	uint64_t actual = fs_get_uint64_by_index(fields, index);
+	return (actual < expected);
+}
+
+static int eval_eq_node(node_t *node, fieldset_t *fields)
+{
+	node_t *literal = node->right_child;
+	int index = node->left_child->value.field.index;
+	char *expected, *actual;
+	switch (literal->type) {
+		case STRING:
+			expected = literal->value.string_literal;
+			actual = fs_get_string_by_index(fields, index);
+			return (strcmp(expected, actual) == 0);
+			break;
+		case INT:
+			return (fs_get_uint64_by_index(fields, index) == literal->value.int_literal);
+			break;
+		default:
+			printf("wat\n");
+			break;
+	}
+	return 0;
+}
+
+static int eval_lt_eq_node(node_t *node, fieldset_t *fields)
+{
+	return !(eval_gt_node(node, fields));
+}
+
+static int eval_gt_eq_node(node_t *node, fieldset_t *fields)
+{
+	return !(eval_lt_node(node, fields));
+}
+
+
+/* Exposed functions */
+
+node_t* make_op_node(enum operation op) 
+{
+	node_t* node = alloc_node();
+	node->type = OP;
+	node->value.op = op;
+	return node;
+}
+
+node_t* make_field_node(char *fieldname) 
+{
+	node_t *node = alloc_node();
+	node->type = FIELD;
+	node->value.field.fieldname = fieldname;
+	return node;
+}
+
+node_t* make_string_node(char *literal)
+{
+	node_t *node = alloc_node();
+	node->type = STRING;
+	node->value.string_literal = literal;
+	return node;
+}
+
+node_t* make_int_node(int literal)
+{
+	node_t *node = alloc_node();
+	node->type = INT;
+	node->value.int_literal = literal;
+	return node;
+}
+
+int evaluate_expression(node_t *root, fieldset_t *fields)
+{
+	if (!root) return 1;
+	switch (root->type) { /* XXX Not sure if runs */
+	case FIELD:
+	case STRING:
+	case INT:
+		return 1;
+	case OP:
+		break;
+	}
+	switch (root->value.op) {
+	case GT:
+		return eval_gt_node(root, fields);
+	case LT:
+		return eval_lt_node(root, fields);
+	case EQ:
+		return eval_eq_node(root, fields);
+	case NEQ:
+		return (!eval_eq_node(root, fields));
+	case LT_EQ:
+		return eval_lt_eq_node(root, fields);
+	case GT_EQ:
+		return eval_gt_eq_node(root, fields);
+	case AND:
+		return (evaluate_expression(root->left_child, fields) 
+			&& evaluate_expression(root->right_child, fields));
+	case OR:
+		return (evaluate_expression(root->left_child, fields) 
+			|| evaluate_expression(root->right_child, fields));	
+	}
+	return 0;
+}
+
+void print_expression(node_t *root)
+{
+	if (!root) return;
+	printf("%s", "( ");
+	print_expression(root->left_child);
+	switch (root->type) {
+		case OP:
+			printf(" %i ", root->value.op);
+			break;
+		case FIELD:
+			printf(" (%s", root->value.field.fieldname);
+			break;
+		case STRING:
+			printf("%s) ", root->value.string_literal);
+			break;
+		case INT:
+			printf(" %llu) ", (long long unsigned) root->value.int_literal);
+			break;
+		default:
+			break;
+	}
+	print_expression(root->right_child);
+	printf("%s", " )");
+}
diff --git a/src/expression.h b/src/expression.h
new file mode 100644
index 0000000..d65668d
--- /dev/null
+++ b/src/expression.h
@@ -0,0 +1,50 @@
+#ifndef ZMAP_TREE_H
+#define ZMAP_TREE_H
+
+#include "fieldset.h"
+
+#include <string.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdint.h>
+
+enum operation {
+	GT, LT, EQ, NEQ, AND, OR, LT_EQ, GT_EQ
+};
+
+enum node_type {
+	OP, FIELD, STRING, INT
+};
+
+struct field_id {
+	int index;
+	char *fieldname;
+};
+
+union node_value {
+	struct field_id field; 
+	char *string_literal;
+	uint64_t int_literal;
+	enum operation op;
+};
+
+typedef struct node {
+	struct node *left_child;
+	struct node *right_child;
+	enum node_type type;
+	union node_value value;
+} node_t;
+
+node_t* make_op_node(enum operation op);
+
+node_t* make_field_node(char *fieldname);
+
+node_t* make_string_node(char *literal);
+
+node_t* make_int_node(int literal);
+
+int evaluate_expression(node_t *root, fieldset_t *fields);
+
+void print_expression(node_t *root);
+
+#endif /* ZMAP_TREE_H */
\ No newline at end of file
diff --git a/src/filter.c b/src/filter.c
new file mode 100644
index 0000000..83bee43
--- /dev/null
+++ b/src/filter.c
@@ -0,0 +1,99 @@
+#include "filter.h"
+#include "state.h"
+#include "lexer.h"
+#include "parser.h"
+#include "expression.h"
+#include "../lib/logger.h"
+
+#include <string.h>
+
+extern int yyparse();
+
+node_t *zfilter;
+
+static int validate_node(node_t *node, fielddefset_t *fields)
+{
+	int index, found = 0;
+	if (node->type == OP) {
+		// These end up getting validated later
+		if (node->value.op == AND || node->value.op == OR) {
+			return 1;
+		}
+		// Comparison node (=, >, <, etc.)
+		// Validate that the field (left child) exists in the fieldset
+		for (index = 0; index < fields->len; index++) {
+			if (fields->fielddefs[index].name) {
+				if (strcmp(fields->fielddefs[index].name, 
+						node->left_child->value.field.fieldname) == 0) {
+					node->left_child->value.field.index = index;
+					found = 1;
+					break;
+				}
+			}
+		}
+		if (!found) {
+			fprintf(stderr, "Field '%s' does not exist\n", 
+					node->left_child->value.field.fieldname);
+			return 0;
+		}
+		// Fieldname is fine, match the type.
+		switch (node->right_child->type) {
+		case STRING:
+			if (strcmp(fields->fielddefs[index].type, "string") == 0) {
+				return 1;
+			} else {
+				fprintf(stderr, "Field '%s' is not of type 'string'\n", 
+						fields->fielddefs[index].name);
+				return 0;
+			}
+		case INT:
+			if (strcmp(fields->fielddefs[index].type, "int") == 0) {
+				return 1;
+			} else {
+				fprintf(stderr, "Field '%s' is not of type 'int'\n", 
+						fields->fielddefs[index].name);
+				return 0;
+			}
+		default:
+			return 0;
+		}
+	} else {
+		// All non-op nodes are valid
+		return 1;
+	}
+	// Didn't validate
+	return 0;
+
+}
+
+int parse_filter_string(char *filter)
+{
+	YY_BUFFER_STATE buffer_state = yy_scan_string(filter);
+	int status = yyparse();
+	yy_delete_buffer(buffer_state);
+	if (status) {
+		// Error
+		log_error("zmap", "Unable to parse filter string: '%s'", filter);
+		return 0;
+	}
+	zconf.filter.expression = zfilter;
+	return 1;
+}
+
+/*
+ * 0     Valid
+ * -1    Invalid Field Name
+ * -2    Type Mismatch
+ */
+int validate_filter(node_t *root, fielddefset_t *fields)
+{
+	int valid;
+	if (!root) {
+		return 1;
+	}
+	valid = validate_node(root, fields);
+	if (!valid) {
+		return 0;
+	}
+	return (validate_filter(root->left_child, fields) && validate_filter(root->right_child, fields));
+}
diff --git a/src/filter.h b/src/filter.h
new file mode 100644
index 0000000..f42af6d
--- /dev/null
+++ b/src/filter.h
@@ -0,0 +1,15 @@
+#ifndef ZMAP_FILTER_H
+#define ZMAP_FILTER_H
+
+#include "expression.h"
+#include "fieldset.h"
+
+struct output_filter {
+	node_t *expression;
+};
+
+int parse_filter_string(char *filter);
+
+int validate_filter(node_t *root, fielddefset_t *fields);
+
+#endif /* ZMAP_FILTER_H */
\ No newline at end of file
diff --git a/src/lexer.l b/src/lexer.l
new file mode 100644
index 0000000..cd884fa
--- /dev/null
+++ b/src/lexer.l
@@ -0,0 +1,26 @@
+%{
+#pragma GCC diagnostic ignored "-Wredundant-decls"
+#include <string.h>
+#include "parser.h"
+
+%}
+
+%option noinput
+%option nounput
+%%
+[0-9]+               yylval.int_literal = (uint64_t) atoll(yytext); return T_NUMBER;
+\n                   /* Ignore end of line */
+[ \t]+               /* Ignore whitespace */
+!=                   return T_NOT_EQ;
+>=                   return T_GT_EQ;
+"<="                 return T_LT_EQ;
+&&                   return T_AND;
+"||"                 return T_OR;
+=                    return '=';
+">"                  return '>';
+"<"                  return '<';
+"("                  return '(';
+")"                  return ')';
+[a-zA-Z][a-zA-Z0-9]+ yylval.string_literal = strdup(yytext); return T_FIELD;
+
+%%
\ No newline at end of file
diff --git a/src/parser.y b/src/parser.y
new file mode 100644
index 0000000..6c44e17
--- /dev/null
+++ b/src/parser.y
@@ -0,0 +1,144 @@
+%{
+#include <stdio.h>
+#include <string.h>
+#include "expression.h"
+#include "lexer.h"
+#include "filter.h"
+
+void yyerror(const char *str)
+{
+	fprintf(stderr,"Parse error: %s\n",str);
+}
+ 
+int yywrap()
+{
+	return 1;
+}
+
+extern node_t *zfilter;
+
+%}
+
+%union {
+	int int_literal;
+	char *string_literal;
+	struct node *expr; 
+}
+
+%token '(' ')' T_AND T_OR
+%token <int_literal> T_NUMBER
+%token <string_literal> T_FIELD
+%token T_NOT_EQ T_GT_EQ '>' '<' '=' T_LT_EQ
+
+%left T_OR
+%left T_AND
+
+%type <expr> filter
+%type <expr> number_filter
+%type <expr> string_filter
+%type <expr> filter_expr
+
+
+%%
+
+expression: filter_expr
+	{
+		zfilter = $1;
+	}
+
+
+filter_expr:
+	filter_expr T_OR filter_expr
+		{
+			$$ = make_op_node(OR);
+			$$->left_child = $1;
+			$$->right_child = $3;
+		}
+	| filter_expr T_AND filter_expr
+		{
+			$$ = make_op_node(AND);
+			$$->left_child = $1;
+			$$->right_child = $3;
+		}
+	| '(' filter_expr ')'
+		{
+			$$ = $2;
+		}
+	| filter
+		{
+			$$ = $1;
+		}
+	;
+
+filter: number_filter 
+		{
+			$$ = $1;
+		}
+	| string_filter
+		{
+			$$ = $1;
+		}
+	;
+
+number_filter: T_FIELD '=' T_NUMBER
+		{
+			$$ = make_op_node(EQ);
+			$$->left_child = make_field_node($1);
+			$$->right_child = make_int_node($3);
+		}	
+	| 
+	T_FIELD '>' T_NUMBER
+		{
+			$$ = make_op_node(GT);
+			$$->left_child = make_field_node($1);
+			$$->right_child = make_int_node($3);
+		}
+	|
+	T_FIELD '<' T_NUMBER
+		{
+			$$ = make_op_node(LT);
+			$$->left_child = make_field_node($1);
+			$$->right_child = make_int_node($3);
+		}
+	|
+	T_FIELD T_NOT_EQ T_NUMBER
+		{
+			$$ = make_op_node(NEQ);
+			$$->left_child = make_field_node($1);
+			$$->right_child = make_int_node($3);
+		}
+	|
+	T_FIELD T_GT_EQ T_NUMBER
+		{
+			$$ = make_op_node(GT_EQ);
+			$$->left_child = make_field_node($1);
+			$$->right_child = make_int_node($3);
+		}
+	|
+	T_FIELD T_LT_EQ T_NUMBER
+		{
+			$$ = make_op_node(LT_EQ);
+			$$->left_child = make_field_node($1);
+			$$->right_child = make_int_node($3);
+		}
+	;
+
+string_filter:
+	T_FIELD '=' T_FIELD
+		{
+			$$ = make_op_node(EQ);
+			$$->left_child = make_field_node($1);
+			$$->right_child = make_string_node($3);
+		}
+	|
+	T_FIELD T_NOT_EQ T_FIELD
+		{
+			$$ = make_op_node(NEQ);
+			$$->left_child = make_field_node($1);
+			$$->right_child = make_string_node($3);
+		}
+	;
+
+%%
+
+
diff --git a/src/recv.c b/src/recv.c
index bb52fcd..6dff8c6 100644
--- a/src/recv.c
+++ b/src/recv.c
@@ -35,6 +35,7 @@
 #include "state.h"
 #include "validate.h"
 #include "fieldset.h"
+#include "expression.h"
 #include "probe_modules/probe_modules.h"
 #include "output_modules/output_modules.h"
 
@@ -129,6 +130,10 @@ void packet_cb(u_char __attribute__((__unused__)) *user,
 	if (is_repeat && zconf.filter_duplicates) {
 		goto cleanup;
 	}
+	if (!evaluate_expression(zconf.filter.expression, fs)) {
+		goto cleanup;
+	}
+
 	o = translate_fieldset(fs, &zconf.fsconf.translation);
 	if (zconf.output_module && zconf.output_module->process_ip) {
 		zconf.output_module->process_ip(o);
diff --git a/src/state.h b/src/state.h
index bb4181a..6ccf4a4 100644
--- a/src/state.h
+++ b/src/state.h
@@ -15,6 +15,7 @@
 
 #include "types.h"
 #include "fieldset.h"
+#include "filter.h"
 
 #ifndef STATE_H
 #define STATE_H
@@ -81,6 +82,7 @@ struct state_conf {
 	int destination_cidrs_len;
 	char *raw_output_fields;
 	char **output_fields;
+	struct output_filter filter;
 	struct fieldset_conf fsconf;
 	int output_fields_len;
 	int dryrun;
diff --git a/src/whitelist b/src/whitelist
new file mode 100644
index 0000000..aab8f1d
--- /dev/null
+++ b/src/whitelist
@@ -0,0 +1 @@
+192.168.1.0/24
diff --git a/src/zmap.c b/src/zmap.c
index b4f564e..7cf1079 100644
--- a/src/zmap.c
+++ b/src/zmap.c
@@ -36,6 +36,7 @@
 #include "state.h"
 #include "monitor.h"
 #include "get_gateway.h"
+#include "filter.h"
 
 #include "output_modules/output_modules.h"
 #include "probe_modules/probe_modules.h"
@@ -514,6 +515,18 @@ int main(int argc, char *argv[])
 			&zconf.fsconf.defs, zconf.output_fields,
 			zconf.output_fields_len);
 
+	// Parse and validate the output filter, if any
+	if (args.output_filter_arg) {
+		// Run it through yyparse to build the expression tree
+		if (!parse_filter_string(args.output_filter_arg)) {
+			log_fatal("zmap", "Unable to parse filter expression");
+		}
+
+		// Check the fields used against the fieldset in use
+		if (!validate_filter(zconf.filter.expression, &zconf.fsconf.defs)) {
+			log_fatal("zmap", "Invalid filter");
+		}
+	}
 
 	SET_BOOL(zconf.dryrun, dryrun);
 	SET_BOOL(zconf.quiet, quiet);
diff --git a/src/zopt.ggo b/src/zopt.ggo
index 48844b6..d1852aa 100644
--- a/src/zopt.ggo
+++ b/src/zopt.ggo
@@ -94,6 +94,9 @@ option "probe-args"		- "Arguments to pass to probe module"
 option "output-args"		- "Arguments to pass to output module"
 	   typestr="args"
 	   optional string
+option "output-filter"          - "Read a file containing an output filter on the first line"
+	   typestr="filename"
+	   optional string
 option "list-output-modules"	- "List available output modules"
            optional
 option "list-probe-modules"	- "List available probe modules"